1. Field of the Invention
The present invention relates generally to a wireless communication system in accordance with Orthogonal Frequency Division Multiplexing, and particularly to a beam formation circuit and an apparatus and a method of receiving radio frequency signals making use of a smart antenna a receiver apparatus.
2. Description of the Related Art
In recent years, with the high speed advent of the wireless communication technique, wireless terminals have been used by end users for the purpose of receiving and transmitting a large amount of information such as images, other types of data, in addition to voices. It is inevitable for high speed transmission of such a large amount of information to treat wide bandwidth signals and also to determine the measures that could be adopted to cope with signal fading and interference. On the other hand, OFDM (Orthogonal Frequency Division Multiplexing) is drawing the attention as a technique of realizing such wide bandwidth signal communications.
The OFDM technique is a block-oriented modulation scheme that maps data symbols onto a plurality of orthogonal sub-carriers separated by a distance and arranged within a limited bandwidth to provide excellent resistance characteristics against the interference due to delay spread through the multipaths. Namely, while the orthogonally encoded data signals can be generated by means of the inverse Fourier transformation or the orthogonally decoded data signals can be extracted by means of the Fourier transformation, it is possible to provide guard intervals in order to remove the influence of the timely delay within the guard intervals. Because of this, tolerability of high speed transmissions to frequency selective fading can be improved.
OFDM having such features has been expected to be applied not only to the cellular systems and the broadcast services but also applied to wireless LANs and so forth.
For example, work is being done to formally standardize the wireless LAN on the basis of OFDM in Japan, Europe and U.S.A. referred to respectively as HiSWANa, HiperLAN and IEEE802.11a. These standards are similar to each other in the physical layer. Particularly, in Europe, the standard is applicable not only to the wireless LAN but also to outdoor usages. In accordance with the respective three standards, there are provided, among from a set of sub-carriers for transmission of packets, particular sub-carriers in preselected positions are used as pilot sub-carriers each of which includes a predetermined bit sequence while all the sub-carriers are provided with a predetermined leading preamble. FIG. 4 is a graphic diagram showing the frequencies of the transmission signals versus time in an exemplary configuration of frames. In this example, there are provided 52 sub-carriers in all each of which has a band width fM and provided with leading short and long preambles. Four pilot sub-carriers are included therein. One OFDM symbol time is 4 μsec.
FIG. 2 and FIG. 3 is a schematic block diagram showing a transmitter apparatus in accordance with a conventional technique OFDM system. FIG. 3 is a schematic block diagram showing a receiver apparatus in accordance with a conventional technique OFDM system. These apparatuses as illustrated respectively in FIG. 2 and FIG. 3 are used in the wireless LAN as described above and have been described in details, for example, in “OFDM for wireless multimedia communications, &QUOT; (Richard Van Nee, Ramjee Prasad, Artech House, 2000).
The processing of the signals by the transmitter apparatus and the receiver apparatus will be briefly explained with reference to FIG. 2 and FIG. 3. At first, in the transmitter apparatus as illustrated in FIG. 2, the data sequences to be transmitted are error corrected and encoded by means of an encoder 601 and then interchanged by means of an interleaver 602. The output signals of the interleaver 602 are converted into multi-level signals in accordance with QAM (quadrature amplitude modulation) by means of a QAM symbol relocation unit 603 followed by inserting pilot signals thereto by means of a pilot signal inserting unit 604.
Then, the QAM data sequences to be transmitted are converted into N paralell data items (N: the number of sub-carriers) by means of a serial-to-parallel converter (S/P converter) 605. The parallel data items are converted into signals in the time domain on the basis of the inverse Fourier transformation by means of the inverse fast Fourier transform unit (IFFT unit) 606 and then converted into serial data sequences by means of the parallel-to-serial converter (P/S converter) 607.
The output signals of the IFFT unit 606 are N sub-carriers as modulated by the corresponding N data items as given from the S/P converter 605. The guard interval inserting circuit 608 serves to put a wave form same as the tail portion of the effective symbols of the OFDM signal on the top of the effective symbols in the serial signals obtained after fast Fourier transformation.
The influence of the timely delay within the guard interval can be removed by the fast Fourier transformation at the receiver side. Then, the transmission signals are converted into analog signals by means of the D/A converter (DAC) 609, orthogonally modulated, frequency converted, filtered, power amplified, treated under other necessary processes, and then wireless transmitted through an antenna element 611.
It is necessary in this type of OFDM system to cope with the interference of delayed signals with delays exceeding the guard interval length, and the interference in the same channel by any other cell and any other system making use of the same frequency band. The effective use of the frequency resources is inevitable as a measure meant to work out the frequency tightness problem. It is effective as a solution of the problem to apply the so-called smart antenna technology to the OFDM system.
The smart antenna technology is effective for increasing the system capacity and the coverage area of a base station and for improving the communication quality by making use of antenna elements whose directivity is variable for the base station (and the mobile station). In this case, with a plurality of antenna elements arranged in an array as illustrated in FIG. 1 rather than a single antenna element having a variable directivity, desired directivity patterns can be formed by weighting the amplitude and the phase for the respective antenna element. With this type of such an adaptive array antenna element capable of electrically controlling the directivity, it is possible not only to direct a peak in an antenna directivity pattern to any desired direction but also to direct a null to interfering waves. Particularly, it is possible to effectively use the frequency resources by forming a plurality of antenna directivity patterns orthogonal to each other (i.e, the null of one pattern to the peak of another) and assigning the same time slot and the same frequency channel to the respective beams corresponding thereto to accomplish the so-called SDMA (Space Division Multiple Access).
A plurality of tapped-delay-lines (TDL) and a plurality of weighting units for the respective antenna elements have to be provided for the smart antenna in order to form appropriate beams carrying wide bandwidth signals. Also, in the case of the OFDM system capable of separating signals in the frequency domain, weighting units provided for each sub-carrier can be used for implementing an equivalent function. In this case, however, a large amount of computation tasks are necessarily required to obtain the weights of all the sub-carriers of the OFDM system. For this reason, the excessive amounts of processing power and computation time become obstacles to the operation of the system, for example, in the case where voice signal processing is required in real time. It becomes in this case therefore necessary to increase the system in circuit size or to make use of a high speed DSP or CPU for removing the obstacles.
Furthermore, since all the sub-carriers are provided with a predetermined bit sequence having a sufficient length, the packet efficiency tends to be reduced (while this is no the case when a blind algorithm is employed). In the case of the wireless communication system as described above making use of a short preamble, it is very difficult to obtain optimal solutions on the basis of known algorithms.